Internally fused capacitors were developed to increase reliability of power capacitors. In this design every capacitor element of a power capacitor is connected through a fuse wire in series.
Capacitor elements consist of few layers of insulating film such as polypropylene, which is wound together with aluminium foil. The aluminium foils work as electrodes and the film layers work as dielectric. The film can have weak spots which over time can lead to a breakdown. In the case of a breakdown a high current flows though the failure point and welds the aluminium foils together such that there is a persistent short circuit in the element.
For an internally fused design at the event of a capacitor element failure, the capacitor elements connected in parallel discharge their energy to the short circuit through the fuse, which normally is enough to achieve a successful current limiting behaviour of the fuse. Current limiting behaviour means that the operated fuse can interrupt the discharge before all parallel energy is dumped into the short circuit spot. One important factor for this function is that the arc created by the operated fuse can expand and be cooled and extinguished by the surrounding insulating materials.
In case of a capacitor element failure, the large discharge current leads to the evaporation of the fuse and then to an arc inside the surrounding material which usually is a fluid such as oil. Due to the good cooling properties of oil, this arc is quenched within a few tens to hundreds of microseconds, leaving an electrical open circuit in-between the foot points where the fuse was connected to. As a consequence, the failing capacitor element is disconnected before damage is large enough to disturb functionality of the remaining capacitor elements.